1. Technical Field
The present invention relates to a DC-DC converter and a power supply device provided with the DC-DC converter.
2. Related Art
Electronic apparatuses such as a car navigation system into which many functions are integrated include a plurality of circuits such as a DVD circuit, a television and radio circuit, and an audio circuit. The individual circuits require different power supply voltages ranging from a relatively high voltage to a low voltage (for example, 9V, 8V, 7V, 5V, 3.3V, and 1.2V).
A device provided with a plurality of DC-DC converter circuits, each of which outputs a single voltage, can be considered as a power supply device for such a multifunction electronic apparatus.
In such a power supply device provided with a plurality of DC-DC converters, if the switching frequencies of the individual DC-DC converters are different from each other, it is difficult to perform noise control. Accordingly, the switching frequencies of individual DC-DC converters are synchronized (see, for example, Japanese Utility Model Registration No. 2583479).
In a power supply device disclosed in Japanese Utility Model Registration No. 2583479, a resistor and a capacitor which are used to determine an oscillation frequency are externally connected to a switching control IC. In order to synchronize the oscillation frequencies of two switching control ICs, a circuit connected between the capacitor connection terminals (CT terminals) of the switching control ICs is prepared. That is, the capacitor connection terminals of the two switching control ICs are connected to each other via a balancing capacitor. The oscillation frequencies of the two switching control ICs are synchronized by causing the balancing capacitor to perform a synchronization function.
In addition, some switching control ICs, each of which requires such a resistor and capacitor to be externally connected thereto, have a function allowing switching control ICs of the same type to be synchronized. Usually, the oscillation circuit of a master switching control IC is operated, and then, the operation of the oscillation circuit of a slave switching control IC is stopped. Subsequently, the slave IC performs a switching operation in synchronization with a signal output from the master IC.
In a power supply device for which voltage outputs ranging from a relatively high voltage to a low voltage are required, if the output voltage of a DC-DC converter is high, that is, if a difference between an input voltage and an output voltage is small, an on duty ratio (duty cycle) increases at the time of switching, and a period in which a current passes through a flywheel diode (the off period of a switching element) becomes short. Consequently, a loss due to a forward voltage drop in the flywheel diode becomes small. In contrast, if the output voltage of a DC-DC converter is low, the period in which a current passes through the flywheel diode becomes long. Consequently, the loss due to a voltage drop becomes an issue.
Accordingly, in a DC-DC converter having a low output voltage, a synchronous rectifier circuit using a switching element (FET) of a low on-resistance is sometimes used instead of a flywheel diode. However, a control IC suitable for the synchronous rectifier circuit, which has to control both a main switching element and a switching element for synchronous rectification, contains a VCO (voltage-controlled oscillator), and is configured to control a switching frequency by applying a control voltage to the VCO. That is, the control IC is not configured to determine an oscillation frequency using a resistor and a capacitor which are externally connected thereto.
Thus, in an arrangement including both a DC-DC converter using a switching control IC that contains a VCO, and a DC-DC converter using a switching control IC to which a resistor and a capacitor are externally connected, a known method disclosed in Japanese Utility Model Registration No. 2583479 cannot be employed.